1. Field of the Invention
The present invention relates to a method and system for fitting image positions.
2. Description of the Related Art
An energy subtraction process and a temporal subtraction process are performed in a digital radiography system using a storage fluorescent material. For example, in the energy subtraction process, two images obtained at different energy levels are subjected to subtraction for displaying a difference image representing the difference between the two images.
In such subtraction processes, the following two methods have been used for fitting positions of two images with respect to each other.
(1) Exclusive markers formed of a X-ray absorbing material such as a heavy metal, are attached to previously determined positions, for example, the top right-hand corner and the bottom left-hand corner of a cassette into which imaging plate (IP) is inserted. By irradiating the IPs with X-rays at different levels through a subject under examination, images Q1 and Q2 of the subject and images m1a, m1b, m2a and m2b of the exclusive markers are formed as shown in FIGS. 1A and 1B. The marker positions on the images Q1 and Q2 are detected to determine the subtraction position in which the two images are subjected to the subtraction process.
(2) Such a subtraction position as to make the entropy of a subtraction image minimum is searched for. More specifically, the subtraction process is performed while shifting the subtraction position by using three parameters, i.e., an amount of lateral shift, an amount of longitudinal shift and an amount of rotation so as to calculate the entropy of the subtraction images. The subtraction position is determined by such three parameters as to make the entropy minimum.
The entropy E is represented as follows: ##EQU1## where Pi is the number of pixels whose gray level of an image is i and N is the number of gray levels of the image.
However, the above two methods have the following problems. That is, in the case of the first method, the exclusive marker images will necessarily be formed on an image of a subject in usual photography by the us of general-purpose X-ray photographing equipment. If the exclusive markers were not used, it would be impossible to fit the positions of two images for the subtraction process.
On the other hand, in the case of the second method, it will take long to calculate the entropy of digital radiography images having a large number of pixels.
In view of the foregoing, there is a demand for an apparatus which allows easy and rapid position fitting of images to be subjected to subtraction.